Aluminum nitride is a promising material in efforts to making more compact, thinner piezoelectric elements for, among other factors, its high fabricability into thin film. However, to utilize aluminum nitride as an piezoelectric element, the aluminum nitride needs to be oriented only along the c-axis. The more c-axis-oriented, the more piezoelectric. In addition, to use an aluminum nitride thin film as a piezoelectric element, the film needs be flanked on top and bottom by electrodes.
Thin films of c-axis-oriented aluminum nitride have been reportedly produced on glass and other substrates by various methods (T. Shiosaki, T. Yamamoto, T. Oda, A. Kawabata, Appl. Phys. Lett., 36 (1980) 643). There are also reports about the production on electrode film. These aluminum nitride films however have great rocking curve full width half maximums (RCFWHMs) of about 3.0° or more, a measure of c-axis orientation, and insufficient piezoelectricity.
Aluminum nitride films with superhigh c-axis orientation (2.5° or less in RCFWHM) are reported by, for example, F. Engelmark, G. F. Iriarte, I. V. Katardjiev, M. Ottosson, P. Muralt, S. Berg, J. Vac. Sci. Technol. A, 19 (2001) 2664. A monocrystal substrate is used as the substrate on which a monocrystal or polycrystalline thin film of aluminum nitride is directly formed. Therefore, no electrode can be provided between the substrate and the aluminum nitride thin film, which renders it difficult to use the film as an piezoelectric element.
Aluminum nitride films show very large inherent internal stress. When fabricated on an electrode, the film may create cracks in the electrodes or peel off the substrate together with the electrodes, raising serious problems in the applying to piezoelectric elements.
The present invention has an objective to provide a high performance piezoelectric element from an aluminum nitride thin film, with no hillocks or cracks, which does not peel off and exhibits superhigh c-axis orientation, by forming a bottom electrode from a W layer on a glass or like cheap substrate with no intervening adhesive layer.
The present invention has another objective to provide a high performance piezoelectric element from an aluminum nitride thin film which exhibits similar superhigh c-axis orientation, by selecting a suitable material for the surface layer of the bottom electrode in the formation of the bottom electrode which is a stack body containing not only a W layer, but also an adhesive layer. Specific stack structures of the bottom electrode will be also proposed.
The present invention provides an easy and cheap method of manufacturing a piezoelectric element based on the foregoing aluminum nitride thin film whereby the aluminum nitride thin film is given superhigh c-axis orientation with the occurrence of hillocks, cracks, and peeling being prevented through the control of particle shape.
The present invention has a further objective to achieve an equivalent level of performance with a cheap glass substrate to that with a monocrystal substrate.
The present invention has still another objective to provide a manufacturing method whereby the bottom electrode is formed by r.f. plasma-assisted sputtering to impart superhigh c-axis orientation to the aluminum nitride thin film.